Polyurethane polymers are well known to impart particularly desirable and high quality film integrity properties and are particularly suitable for protective coatings or paint coatings applied to substrates. The most preferred polyurethanes are homopolymer polyurethane polymers consisting essentially of repeating urethane units and such polymers exhibit much improved film properties over modified polyurethanes such as oil-modified air drying polyurethanes or similar polyurethane copolymers. In essence, the polyurethane homopolymer urethane readily exhibits the best film properties but often must be modified in practice with other polymer chain components such as fatty acids or hydrocarbon polymeric chains so as to obtain flexibility and proper adhesion of the film to the substrate. More recently, the need for water soluble polyurethane coatings has been suggested for environmental reasons to avoid organic solvent systems. However, water miscible polyurethane coatings have been difficult to produce as well as deficient in film integrity properties when compared to solvent based polyurethane coatings. Soluble polyurethane coatings often exhibit poor water resistance, poor humidity resistance, poor chemical resistance, and poor abrasion resistance. U.S. Pat. No. 3,773,729 proposes a water soluble polyurethane copolymer primarily based upon an internal nonurethane polymer of a high molecular weight hydrocarbon produced by co-reaction of ethylenic unsaturated monomers wherein the high molecular weight hydrocarbon chain is capped by very minor weight amounts of diisocyanate to form a urethane modified hydrocarbon dispersed in water.
It now has been found that an unmodified water soluble polyurethane homopolymer of urethane polymeric units can be produced by co-reacting a blocked di- or triisocyanate, a polyol containing two or more hydroxyls, a hydroxy-acid material containing at least one reactive hydroxy group and at least one nonreactive carboxy group to form a polyurethane polymer which reacted further with ammonia or an organic amine to solubilize the polyurethane polymer within water. The blocked polyurethane polymer can then be applied to a substrate and cured by heat. The heat cure co-reacts the free hydroxy and the formerly blocked isocyanate group on the polyurethane polymer to form a cured internally cross-linked polymer. The protective coating films thus produced contain a high percentage of urethane linkages (as high as 37% by polymer weight) thereby exhibiting extraordinarily high film hardness and film abrasion resistance as well as excellent chemical and solvent resistance. Excellent stability in water as well as excellent film adhesion to substrates is readily achieved. The polyurethane polymer is very homogeneous consisting essentially of repeating urethane linkages. These and many other advantages are achieved by the heat curable water dispersible polyurethane homopolymer of this invention.